PCs that do not power up


1

116e32s

Since the late 90s, I have come across quite a few PCs that do not power on.
I have checked out the power supply, the power switch, removed everything
except one stick of RAM, and they still lie dead.
I would expect there was a simple on-off connection to fire up the ATX power
supply, but there must be some logic circuit on the mainboard that checks
if a CPU is plugged in or whatever, and that is frizzed out.
Those were various brands of mainboard, the last one I had was Gigabyte.
 
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P

Paul

Since the late 90s, I have come across quite a few PCs that do not power on.
I have checked out the power supply, the power switch, removed everything
except one stick of RAM, and they still lie dead.
I would expect there was a simple on-off connection to fire up the ATX power
supply, but there must be some logic circuit on the mainboard that checks
if a CPU is plugged in or whatever, and that is frizzed out.
Those were various brands of mainboard, the last one I had was Gigabyte.

You don't need a CPU, to turn on a PC power supply.

If you cable up a bare motherboard to a PSU, connect the
power button to the correct two pins on the front panel
header, you can turn it on. For example, on one of my
older PCs, the VID pin coding table for the CPU has an
"all 1's" table entry where the output of VCore is set
to zero. When the CPU is not present in the socket, the
VCore is turned off so the socket is not powered. The rest
of the motherboard remains powered and operational. The
clock generator delivers nominal clocks, from reset (so all
of the logic can initialize without a CPU present). The BIOS
can't run without a CPU, but the dumb power logic works
just fine without CPU intervention.

The motherboard converts an "ON pulse" from the front panel
button, into a steady ON signal on the PS_ON# wire of the
main cable. That's how the second half of the power supply
is turned on. The first half of the power supply is always
running, and it makes +5VSB. That +5VSB powers the motherboard logic
that sits there looking for a pulse from the front panel button.
The front panel button is a momentary SPST switch, not a rocker
or toggle or bat handle switch which would continuously indicate
the desired state. One pulse means ON, a second pulse means OFF.

That's called "soft power". Power control done with logic gates,
capable of turning on power conversion with a relatively weak or
low power signal.

You have to work your way through that logic, to figure out
why a PC won't start. Checking the level carefully with an
analog meter, on the PS_ON# wire, is part of that. (That's because
the ATX PSU does analog measurement of the PS_ON# wire and
doesn't use a purely digital method - a power supply can be
"half-ON" as a result.)

The motherboard logic is rather complicated, in that the logic
tree also includes lots of "Wake" signals. There is also back
feed protection features (analog power cutoff) in the design,
which complicate the time and voltage analysis of what happens
at start up. For example, there are some motherboards that
pretend "I'm still awake" in terms of their internal state, and
then they don't generate a RESET pulse to kick off the boot process.
And this is a failure in the sequencing logic on the motherboard.
On one computer, this was caused by current leakage coming from
the LCD monitor. If you pulled out the LCD plug, pushing the
button on the front of the PC then worked properly. Naturally,
the user discovering this, was pissed.

So the power control uses logic gates, and enough of the
motherboard works, that you should still be able to
turn an ATX supply on and off, using motherboard, PSU,
and front panel POWER button.

This diagram may not be completely accurate, but it's intended
to show how the momentary pulse from the front panel switch
(active low), makes the PS_ON# go active low in response.
The PSU turns on, when PS_ON# is around 0.7V or less.
And the PS_ON# signal type is open collector, so you
can connect a bypass jumper to ground to the signal,
and cause the supply to come on (not really all that
good an idea, but I have to mention it). Notice that
PS_ON# on the main cable, stays at logic 0 for the
entire computing session.
____ ________________________ _______
Front button |___| |___|
________ _______
PS_ON# |____________________________|

If you defeat PS_ON#, by grounding pin 14 on the main 20 pin
cable, it does turn on the PC. But that also defeats the
automatic turn-off functions, such as overheat protection.
If you use a grounding jumper on 14, the PC could burn up,
even through THERMTRIP is saying "turn off please". The
PC would be defenseless against faults with the jumper
in place.

Paul
 
S

s|b

Since the late 90s, I have come across quite a few PCs that do not power on.
I have checked out the power supply, the power switch, removed everything
except one stick of RAM, and they still lie dead.

I had this happen to me, this weekend. At first, the power went on after
I pressed the power button several times. Then it didn't start at all.
The led on my motherboard was lit, so there was still power. Then it hit
me, change the battery on the motherboard. After I did this all was
fine...
 
M

Michael Black

I had this happen to me, this weekend. At first, the power went on after
I pressed the power button several times. Then it didn't start at all.
The led on my motherboard was lit, so there was still power. Then it hit
me, change the battery on the motherboard. After I did this all was
fine...
Is that consistent, the battery needed to power up the motherboard?

I know in the Mac world, there was endless arguments about this, because
some Macs needed the battery to start up, others didn't.

Michael
 
P

Paul

Michael said:
Is that consistent, the battery needed to power up the motherboard?

I know in the Mac world, there was endless arguments about this, because
some Macs needed the battery to start up, others didn't.

Michael

I have a Mac G4 Quad Nostril machine, with a bad lithium
battery (doesn't keep time), and it starts just fine.
Frankly, I was surprised that it did start. My previous
Mac (PTP250), would have dropped dead if that happened.
Because it was keeping some OF parameters, and a loss of
battery would lose those. There was some goofy procedure
to restore them (which I've likely lost).

*******

The SuperI/O chip has a connection to VBAT for monitoring
purposes. That's about the only other connection, other
than the ORing diode power that runs to the Southbridge
SuperI/O CMOS well. It's unclear why some PCs cannot
start when VBAT is exactly zero. Maybe I should draw a picture.

+5VSB --- regulator --- +3VSB ---- ORing diode -----+
+------- CMOS in Southbridge
CR2032 -+- 1K ohm resistor ------- Oring diode -----+ Power control logic
| CMOS RAM
| RTC digital watch
|
+------ VBAT on SuperI/O for monitoring
by taking voltage readings when the
PC is running ?

The +5VSB (comes from your ATX supply, should have been
enough to always make the PC start. When a PC will not
react to the Power button, and VBAT is zero volts, the
ORing diode on the bottom leg is reverse biased,
and no current is flowing in that leg. It all should
have worked. The top leg should have delivered a bit
more than 3V to the power input on the Southbridge.

I can only guess, that the SuperI/O VBAT input pad,
has some logic connection to other parts of the
PS_ON# or waking logic. And that's how it gets
gated off. But, I've never seen any docs to that
effect. And for that matter, I've never seen
any proof that you can actually read out VBAT
as a channel, on the 8 or 9 channel ADC inside the SuperI/O.
All we know, is there is a VBAT input, and it is there
purely for some kind of monitoring function. It isn't spelled
out, how it might behave at startup time.

Paul
 
F

Flasherly

I had this happen to me, this weekend. At first, the power went on after
I pressed the power button several times. Then it didn't start at all.
The led on my motherboard was lit, so there was still power. Then it hit
me, change the battery on the motherboard. After I did this all was
fine...

Try, next time, removing the PS power cord or using the PS's switch to
turn it off. Give it 10 seconds and turn it back on to restore wall
continuity. Then turn it on normally from the front, shorting the
contact from the case Power-On two-lead to the MB switch-block logic.
ATX PS logic - I've seen some that get messed up with whether house
current was present prior;- also similar, symptomatic is whether a
short press to power-off takes or requires further holding in the
power on/off button an inordinate time to shut down.
 
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S

s|b

Is that consistent, the battery needed to power up the motherboard?

I know in the Mac world, there was endless arguments about this, because
some Macs needed the battery to start up, others didn't.

I had a Performa 475 ages ago and I remember it doing nothing, didn't
boot. Went to the shop, they replaced the battery and the problem was
solved.
 

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